Sideboard



FIG. 1 is a left front perspective view of a sideboard showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a side view, the opposite side being a mirror image thereof; and,

FIG. 4 is a top view.

The broken lines showing the knob hardware is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a sideboard, as shown and described. 